It is already known that pumping of liquids having a high gas content may not be carried out without simultaneous gas discharge, because the gas will separate and accumulate around the center of the rotor of the pump and form a bubble which tends to grow and eventually will fill the entire inlet opening of the pump. This results in a considerable decrease of efficiency and will cause vibration of the equipment and in the worst case leads to the interruption of the pumping action all together. This problem appears to be especially severe in centrifugal pumps which have been used for decades, for example, for pumping low consistency fiber suspensions in the wood processing industry. Various attempts have been made to solve these problems by discharging the gas bubble from the pump. Today gas is discharged in known and used apparatus either by drawing gas with suction through a pipe which extends into the hub of the impeller located in the center of the suction opening of the pump, or by drawing the gas through the hollow shaft of the impeller, or by arranging at least one hole in the impeller through which the gas is drawn toward the back side of the impeller and further away therefrom.
All the apparatus operate satisfactorily when the medium being pumped is liquid and substantially free from solids. Problems arise only when the medium includes solid particles, such as fibers, threads, etc. In such cases these particles prevent the ducts from remaining clear and open, which again is a necessity for the proper operation of the pump.
There are several known ways by which the disadvantage and risk factors caused by these impurities may be eliminated or minimized. The simplest way is to provide a sufficiently large gas discharge duct so that clogging is out of the question. Other alternatives are, for example, the provision of different blade wheel arrangements at the back side of the impeller. Often radial vanes are arranged on the back surface of the impeller, the purpose of which vanes is to pump the medium which has flown with the gas through the gas discharge openings of the impeller to the outer rim or periphery of the impeller and from its clearance back to the liquid flow. The ultimate purpose of the vanes behind the impeller is to balance the axial forces of the pump which is preferably accomplished when the number of back vanes is equal to the number of the pumping vanes. In some cases a separate pumping arrangement is used having the same purpose as above mentioned, but which is mounted further behind the impeller by means of a blade wheel mounted on the shaft of the impeller. This blade wheel rotates in its own chamber thereby separating the liquid flowing with the gas and guiding it to the outer periphery of the chamber so that the gas can be drawn away by suction from the center of the chamber. The medium together with the impurities accumulated at the outer periphery of the chamber is guided via a separate duct either to the suction or to discharge side of the pump. As stated, all disclosed apparatus operate satisfactorily only when a limited amount of impurities is present in the liquid.
It is also possible to adjust the apparatus to operate relatively reliably also with liquids containing a substantial amount of solids, for example, fiber suspensions of the pulp and paper industry. In that case it is, however, necessary to yield in the gas discharge ability of the pump, since it is essential that no or hardly any fibers enter the gas discharge duct or come into contact with the vacuum pump communicating therewith. Thus gaseous fiber suspension present behind the impeller is, as a precaution, generally fed back to the main flow. On the other hand, it is known that the presence of gas in the fiber suspension is a negative factor in the pulp treatment process which should be eliminated as far as possible. It is therefore a waste of existing advantages to feed the once-separated gas back to the pulp circulation. On the other hand, it is also a waste of pulp to separate the pulp flow together with the gas from the pulp circulation by discharging the gaseous pulp as a secondary flow from the pump.
The purpose of the present invention is therefore to utilize the capability of a centrifugal pump most efficiently for separating gas from a liquid by discharging the gas from the pump by simple and operationally safe means. The only precondition is to operate without the risk of impurities present in the liquid, i.e. solids, such as threads, fibers, etc., to clog the gas discharge system.
Pending U.S. Pat. appln. No. 216,009, now U.S. Pat. No. 4,921,400, discloses a method of ensuring that the fibers of the suspension cannot clog the gas discharge system or the vacuum pump communicating therewith even in the case of pumping fiber suspensions of the pulp and paper industry. In that application a filter surface or the like is arranged in the flow passage of the gas being discharged prior to entry thereof into the vacuum pump used in the process. The disclosed filter surface prevents the fibers from entering the gas discharge system.
U.S. Pat. No. 4,673,330 discloses a method of controlling the operation of a centrifugal pump by adjusting the size of the gas bubble generated in front of the pump impeller. The device in accordance with that publication comprises a plurality of electric sensors arranged radially on the rear wall of the pump housing behind the impeller. The sensors measure the size of the gas bubble generated between the impeller and the rear wall on the basis of the varying ability of liquid and gas to conduct electricity.
It is noted in that publication that neither the medium between the vanes of the impeller nor the gas bubble inside the medium are evenly round, but that the boundary surface therebetween is to some extent serrate in such a way that each foil in a way pushes the medium layer in front of it and the medium layer tends to move towards the outer periphery of the pump due to the centrifugal force. However, for some unexplained reason that portion of the medium which is in contact with the front surface of the vane pushing the pulp is closer to the center of the impeller. Such irregularity is present not only at the pumping vanes, but also at the so-called rear or back vanes radially arranged behind the impeller.